DESCRIPTION: (Applicant's Abstract) Stem cell therapies play a major and increasing role in cancer therapies. If we were able to isolate repopulating stem cells, expand them in vitro and genetically manipulate them prior to reinfusion, we would be able to explore many avenues of protective therapies, including expansion of stem cells for multi-cycle chemotherapy, protection of stem cells with chemotherapy resistance genes, and improvement in stem cell quality with DNA repair genes. Published studies indicate that the homeobox gene HOXB4 is expressed in primitive hematopoietic cells, and that its enforced expression induces stem cell symmetric cycling. Our preliminary data indicates that we have isolated the human HOXB4 gene and its promoter, and that we have isolated at least two transcription factors that activate this promoter in both leukemic cells and normal CD34+ cells. In addition, we have found that CD34+ cells activating a HOXB4 expression construct are substantially enriched in long term culture initiating cells (LTCIC). This proposal seeks to identify and stimulate the fraction of stem cells that are capable of self-renewal, as illustrated by their ability to transcribe the homeobox gene HOXB4. In particular, we propose to: 1) Test the Ability of the Activated HOXB4 Promoter to NOD/SCID Repopulating Cells following Xenotransplantation; 2) Identify the Transcription Factors Activating the HOXB4 Promoter at the HxRE-1 (NF-1) Site; and 3) Identify Additional More Distant Genetic Elements That Enhance the Activity of the HOXB4 Promoter In Vivo. Integrated together, these aims will define a new paradigm for the molecular identification and manipulation of stem cells for hematopoietic support and genetic modification. If successful, this model should be directly translatable to clinical practice in the future.